The metabolic fate of drugs in animals and man is being studied in order to relate pharmacological action to molecular events and clinical efficacy. To detect and assay drugs or their effects of neurotransmitters, the following isotopically labelled compounds have been synthesized: imipramine-d3, desipramine-d3, bromocryptine- Br82, 3-methoxy-4-hydroxymandelic acid-d, 3-hydroxy-4-methoxymandelic acid (dO and d3), 3,4 dihydroxyphenylacetic acid-d5, and 3-methoxy-4-hydroxyphenylglycol (d3 and d5). Methods for assay of desipramine and imipramine in physiological fluids (blood, CSF, saliva) have been described. Studies on the distribution and elimination of bromocryptine in primates are in progress, with particular emphasis on the metabolic fate of this compound.